Monday, Nov 3
Melvin Stephens, Estimating Program Benefits
Diana, J.S., S. Maruca, and Bobbi Low. 2006. "Do increasing cormorant populations threaten sportfishes in the great lakes? A case study in Lake Huron." Journal of Great Lakes Research, 32(2): 306-320.
After near extirpation earlier this century in the Great Lakes, double-crested cormorant (Phalacrocorax auritus) populations reached all-time highs, raising concerns among sport anglers and fisheries managers, who see cormorants as a significant source of mortality for yellow perch (Perca flavescens), whose populations are perceived to have declined in concert with cormorant increases. We partitioned age-specific perch mortality for 1995 into that caused by (1) cormorant predation, (2) angler harvest, and (3) other sources. Cormorant consumption of perch was calculated by simultaneously estimating diet composition (using stomach analysis and number of foraging cormorants from aerial and ground surveys), and daily consumption (using energetic values from the literature). Consumption of perch by cormorants was heaviest during April and May (48% of the diet by weight) when perch spawn, but yellow perch was not an important prey species overall (11%). Primary prey groups were alewife (47%) and sticklebacks (15%). Cormorants consumed 11,400 kg of perch, mostly young fish (ages 1-3), but mortality caused by cormorant predation was estimated at <= 6.3% of mortality for all ages of perch. Neither cormorants nor anglers contributed more than one-fourth of the total annual mortality of any age class of perch. Although cormorants were highly visible predators of perch in Les Cheneaux Islands area, they did not represent a major source of mortality, in 1995. Impact of cormorants on perch may vary from year to year, and may relate to fluctuations in timing of the birds' migration relative to perch spawning. However, we conclude that overall impact is not significant, and control measures to limit their populations were not warranted at the time of this study.